Garden drainage for slopes

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Sloping gardens are a very typical in hilly areas and regions of topographical undulation.

Although there are some advantages of having a sloping garden there are also many disadvantages. This can be particularly so when it comes to landscape drainage.

Garden hydrology in hilly regions can be complex with slopes distorting both catchment and surface run-off acceleration.  On slopes it is very typical for your gardens drainage to be affected by surrounding properties.

​The most problematic garden drainage on slopes is when water flows towards the home. In this article I will provide a summary of typical garden drainage problems on slopes and how they can be resolved. 
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​Why do you need to install drainage on slopes? 

Sloping ground can greatly accelerate the velocity of storm water runoff in times of heavy rain.

This can lead to flooding events which in extreme cases can cause damage and financial loss. Land on a gradient is also much more susceptible to movement and subsidence. Seasonal moisture changes on such sites can help contribute to such movements.

​Consequently, providing adequate landscape drainage to sloping gardens is always advised. This is particularly the case on sites with extreme bogginess and moisture fluctuations throughout the year. 
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​How do you drain a sloping garden?

To drain a sloping garden effectively you will firstly need to diagnose the source of your particular issue. 

Most drainage problems on sloping sites are due to topographical funnel points. This is where surrounding landform channels and directs run-off into a specific area.

​These wet zones can become particularly boggy and saturated. 
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Landscape features such tree lines, boundary walls and buildings can also act as a barrier to water travelling downhill.

​In such cases horizontal drainage channels must be installed to collect water and direct it away in an orderly fashion. These can include, French drains, channel drains, swales and other landscape drainage installations.

I will describe these in more detail below:

​6 Drainage installations for sloping gardens 

​Channel drains

Channel drains are linear drainage channels which slot together to form a continuous surface collection drain. These channels are typically topped with a robust drainage grill which allows water into the drain.

Channel drains are typically installed into paved surfaces where surface water can be captured. On sloping sites these drains can be installed along garden edging, retaining walls and patios.

​The water can then be piped downhill to a collection vessel or drainage network. 
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​French drains

​French drains are long trenches filled with free draining gravel encasing a perforated drainage pipe.

These drainage channels can not only direct water flows but draw water from the surrounding ground. In sloping gardens French drains are laid along the width of a slope to collect all downward moving water.

​French drains are very effective at draining lawns and low lying boggy areas. These drainage channels can be plumbed into soak-aways. 
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​Garden levelling 

​One of the most effective ways to mitigate drainage problems on sloping sites is by levelling.

Levelling upon slopes is almost always the most expensive option, especially where terracing is involved. However terracing a whole slope is not necessary to improve its drainage. Simply by flushing up the levels to the existing ground will make a significant improvement.

​Scraping off high areas into low areas can create a more consistent slope which allows water to escape evenly. 

​Retaining walls

​Retaining walls are almost always associated with raised beds, and terracing slopes. However retaining walls can also be utilised strategically for garden drainage on gradients.

Walls can be positioned to make changes within the slopes gradient. Subsequent gradient manipulation can help funnel surface water where it is less likely to affect lawns and flower borders. 

​Grated channel drains can also be installed to the top and base of retaining walls to capture storm water. 

​Mounds

​Mounded soil can create linear bunds of ground which help to redirect surface water on slopes.

Mounds can be particularly effective at steering water around a property or away from a specific area. These types of damming mounds are very effective combined with French drains.

​French drains can be installed along the line of the mound to collect excess surface water. The water can then be directed away or to a soak-away system. 
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​Moats & Swales

Moats and swales are shallow ditches and landscape depressions which collect water during heavy rain.

Traditionally moats would be larger and hold standing water where swales would normally only fill during wet weather. Nonetheless moats and swales can be excavated along sloping ground to help channel and redirect water.

​Contouring such channels can help to reduce flash flooding and garden drainage problems on slopes. 
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​Thank you for reading our article on garden drainage for slopes. If you require garden drainage services do not hesitate to contact us.

Based in Amersham we cover Buckinghamshire and neighbouring counties. We have also just opened a new office in Norfolk.

​Below I will link to some of my other landscape drainage articles you may find useful.  

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